29th International Cosmic Ray Conference Pune (2005) 2, 81-84

A study of sidereal anisotropy of Cosmic rays with GRAPES-3
Muon Telescopes
H.Kojimac, S.K. Guptab, Y. Hayashia, N. Itoa, A. Jainb, A.V. Johnb, S. Karthikeyanb,
S.Kawakamia, K.Matsumotoa, Y.Matsumotoa, T.Matsuyamaa, D.K.Mohantyb,P.K.Mohantyb, S.D.Morrisb, T.Nonakaa, T.Okudaa, A.Oshimaa, B.S.Raob,
K.C.Ravindranb, M.Sasanoa, K.Sivaprasadb, B.V.Sreekantanb, H.Tanakaa, S.C.Tonwarb,
K.Viswanathanb and T. Yoshikoshia(a) Graduate School of Science, Osaka City University, Osaka 558-8585, Japan(b) Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005, India(c) Nagoya Women’s University, Nagoya 467-8610, Japan

Presenter: H. Kojima (hkojima@nagoya-wu.ac.jp), jap-kojima-H-abs1-sh32-oral

We have observed the cosmic rays with GRAPES-3 multi-directional muon telescope (total area of 560m2)
at Ooty ( 11.4 N latitude and 76.7 E longitude, 2200m altitude ), India. We have analyzed muon intensity
variation over a period of 5 years (2000-2004). The results obtained on with sidereal anisotropy aresummarized. Currently the so called “Tail-IN and Loss-cone” model proposed by the Nagashima et al.
(1998) is largely describes the sidereal anisotropy. Our analysis shows the existence of Tail-IN , but absenceof Loss-cone feature.

1. Introduction
Observation of sidereal time anisotropy of the cosmic ray is an effective method for determining theelectromagnetic environment in the interstellar space near boundary of the heliosphere which is notunderstood very well. Nagashima et al. have studied sidereal time variation for various energies of cosmicrays. A number of experiment such as air shower measurements at Mt. Norikura (2750 m a.s.l), muonintensity measurements at Nagoya (sea level), muon measurements at Sakashita (underground) and Hobart(underground, Australia) [3]. These meaurements have indicated the existence of two kinds of sidereal timeanisotropies, namely an intensity excess at sidereal time of 6 hour RA and deficit at 12 hour RA. These arerespectively called Tail-IN (TI) and Loss-cone (LC) anisotropies. In addition, Hall et al. (1999) haveanalyzed the data from many muon stations all over the world (sea level and underground) and haveobtained results similar to that of Nagashima et al. and have displayed their results in the form of a contour
map on the celestial sphere [4].
We have been operating a large area muon telescope (560 m2) ar part of the GRAPES-3 air shower
experiment for the past 5 years and measure the incident angle of muons above 1 GeV which correspond to a
primary cosmic ray of median energy 60 GeV. Five years of data, from January, 2000 to December, 2004
were analyzed to obtain the sidereal time variation of primary cosmic rays. Our results have been comparedwith those of Nagashima et al. [3]. and Hall et al. [4]. Although the TI anisotropy was observed in our data,
the LC anisotropy was not seen clearly. This indicates that there are different causes for these twoanisotropies. The results are shown and discussed in detail in the following section.

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2. Analysis and results
Using Using data of five years (2000-2004) from GRAPES-3 muon telescopes, we obtained the siderealtime variation of primary cosmic rays. Our muon telescopes are able to measure the incidence direction of
the cosmic ray with rather good accuracy (7 degrees). The data were classified according to the direction(polarity) (Toward and Away) of IMF. The polarity of each day was determined from the satellite data of
ACE level 2. Calculating the 72 hour running average we adopted the value of 22h UT for the polarity of
the corresponding day. The muon telescope consists of a total of 16 modules (each module has an effective
area of 35 m2 , total area of 560 m2 ) resulting in smallest statistical error among all instruments operating in
the world. The data for each direction is summed for one hour and the resulting counting rate is normalizedby average value for the day and also corrected for barometric pressure [6]. In addition, in order to remove
various systematic errors, we adopted the so called E-W method (refer to [2] for details) to obtain thesidereal time variation [5]. For a median energy of 60 GeV the sidereal time variation, there would becontamination due to the daily variation of pseudo sidereal time anisotropy called Swinson flow [1].
This anisotropy arises due to the gradient of cosmic rays in the vicinity of the Earth and the polarity(Toward, Away) of IMF. The amplitude (about 0.07%) of anisotropy due to Swinson flow is larger than theexpected genuine sidereal time anisotropy (about 0.03%). The average anisotropy by Swinson flow should
show the same variation with a phase difference of 12 hours with respect to the polarity of IMF. So, if wetake average of the intensity for both polarities (T+A)/2, then pseudo anisotropy due to Swinson flow can becancelled and only a genuine anisotropy would be left in the data.
In case of (T-A)/2 thie effect of the original Swinson flow would be enhanced by a factor of 2. These two
sets of data namely, (T-A)/2 and (T+A)/2 observed by the GRAPES-3 muon telescopes, are shown in Fig. 1and Fig. 2 for 5 years from 2000 to 2004. Each panel shows the sidereal time variation in an individual year.
As one can see in these two figures that for each year the value fluctuates considerably. In the case of (TA)/2 “Swinson flow” intensity variation should follow a sinusoidal wave with a maximum at a phase at 18hours and a minimum at 6 hours for a long-term time average. The daily variation for each year is distorted
in due to effect of “Swinson flow” in the case of single year data. It implies that single year data cannot be
used to judge the genuine sidereal anisotropy. Therefore avaerage of five years of data is taken and is shownfor (T-A)/2 in Fig. 3 and (T+A)/2 in Fig. 4. As can be seen in Fig. 3, (T-A)/2, the phase of the minimum
appears around 6 hours and the maximum near 18 hours. Thus averaging over five years has resulted ineffect of Swinson flow being smoothed into a sine wave and the effect of pseudo sidereal anisotropy could
be largely cancelled. Same is true for the genuine sidereal anisotropy as well. As seen in Fig. 4 for (T+A)/2,there exist a clear excess at 6 hours, but no other clear excess or deficit can be seen in present analysis.

3. Conclusions
Present analysis shows the existence of Tail-IN excess, but the Loss-cone deficit is not clearly seen. Since
three years out of total of five years observation period happen to fall in the maximum phase of Solar
activity, a strong disturbance due to this Solar activity might have been hiding the Loss-cone typeanisotropy. The energy of the cosmic rays connected with Tail-IN is considered to be lower than Loss-cone.
The fact that the anisotropy is seen only at low energy in present analysis is somewhat difficult tounderstand. One of the possible explanation for this kind of Tail-IN anisotropy could be the difference
between the “Nose” and “Tail” structure of the heliosphere which exists considerably deeper in theheliosphere than usually expected.

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4. Acknowledgements
We are happy to acknowledge valuable contributions of D.B. Arjunan, A.A. Basha, G.P. Francis, I.M.
Haroon, V. Jeyakumar,K. Manjunath, B. Rajesh, K.Ramadass, C. Ravindran, and V. Viswanathan during theinstallation, operation and maintenance of providing site facilities for the GRAPES -3 array are gratefully
acknowledged.

References[1] Swinson, J.D.B., J. Geophys. Res., 745591,1969[2] Nagashima, K.,et al., Nuovo Cimento, C(6), 550. 1983a[3] Nagashima, K., Fujimoto, K., and Jacklyn, R.M., J. Geophys. Res., 103, A8,17429., 1998
[4] Hall D.L.,et al., J. ,Geophys. Res. 104, 6737, 1999
[5] Kojima H.,et al., Proc 28th Int Cosmic Ray Conf., Tsukuba,Japan, 7, 3957–3960.,
2003[6] Hayashi Y.,et al., Nucl. Instr. & Meth. A 545, 643 (2005).